How to use Texture Paint on two and more different objects at the same time
This is actually a fairly simple process:


Create your two separate objects
Join them together as a single object using CTRLJ
UV unwrap them so that their UVs don't overlap
Paint your textures
Select the mesh for one object while in edit mode and press P to separate it back into another object.


You can keep a copy of the join mesh around as well if you want to go back and tweak your textures.